1. Field of the Invention
The present invention relates to wireline communications and more particularly to echo cancellation.
2. State of the Art
In wireline communications, echo cancellation is used to remove from a received signal a portion of the signal attributable to reflection of a transmitted signal. The bulk of an echo signal can be cancelled using linear techniques. Linear echo cancellation, however, may still leave substantial residual echo attributable to non-linear distortions within the communications channel. One significant source of non-linear distortion is a transmit digital-to-analog converter (DAC). DAC linearity is specified in terms of bits. For example, a 16-bit DAC may have only 15-bit linearity.
Various approaches to non-linear echo cancellation have been attempted with varying degrees of success. U.S. Pat. No. 5,148,427, for example, describes an arrangement using a fast transversal-filter-based linear echo canceller and a slow, lookup-table (LUT)-based non-linear echo canceller. The LUT is shown as consisting of five 16-word RAMs, for a total of 80 words of storage. For a particular sequence of data symbols, data words from respective locations within each RAM are summed together to form an estimate of a non-linear echo contribution for that data sequence.
U.S. Pat. No. 4,669,116 describes a quite different arrangement in which a conventional transversal-filter-based echo canceller is adapted to cancel both linear and non-linear distortion by determining selected taps corresponding to prominent coefficients of the nonlinearity and adopting for those taps a non-linear structure to which a suitably calculated tap weight is applied.
Nevertheless, there remains a need for a technique of non-linear echo cancellation that is both simple and effective.
The present invention, generally speaking, provides for cancellation of non-linear distortions within the echo path of a communications system by characterizing the non-linearity, performing digital processing of a data signal to cause substantially the same nonlinearity to be applied to the data signal, and inputting a resulting data signal to a non-linear echo-cancellation path. In an exemplary embodiment, the non-linear echo-cancellation path includes as a nonlinear echo canceller a transversal filter or the like. A separate linear echo cancellation path is also provided. Training of the nonlinear echo canceller follows training of the linear echo canceller. This technique is particularly applicable to cancelling the effects of DAC nonlinearity, which can be readily characterized. Using this technique, cancellation improvement of about 3dB can readily be obtained. Alternatively, instead of achieving a lower residual echo floor, the linearity requirements for the transmit DAC can be relaxed. A lower number of bits of precision allows for lower DAC power consumption. The technique is particularly applicable to DSL applications.